I inherited a main panel box from a homeowner that wasn't always fond of meeting the required NEC codes. As such, there are some oddities in the panel. One such oddity is the fact that he installed a couple of tandem circuit breakers for use in 120/240 circuits.

Specifically, there are two Murray MH-T 30-amp breakers. These are tandem breakers and not "slimline" or "thin" breakers -- the two breaker switches on each are not connected in any way. One of these breakers is used for the clothes dryer and the other for an electric water heater.

In the case of the dryer circuit, there's one 10/3 NM wire, with the hot black attached to one of the sides of the tandem breaker and the hot red attached to the other side. The neutral is connected to the neutral bar. In the water heater case, there's a 10/2, with both the white and black attached to their respective sides of the tandem breaker.

I want to stress that this absolutely is working. Both the dryer and water heater were wired this way well before I moved in and it's only now, years later, that I noticed what was going on. It's worked fine all along.

This seems all sorts of wrong, though. It's still a single pole breaker, even if there are two 120v circuits coming off it. It seems like if this was okay, that I'd hear about using a tandem breaker as a space-saving slim 220v breaker... but I've seem nothing.

So, what's wrong with doing this? What's the ramifications if I don't change these two out with traditional double-pole breakers?

EDIT: Added photo of the breakers in question:

The very top is a double-pole 20-amp breaker that's fine. The first 30-amp tandem is the dryer breaker and the second is the water heater.

EDIT 2: Added photo of the dryer breaker popped off the panel:

You can clearly see the two buses and that there doesn't appear to be any shenanigans connecting the two of them. I removed the top double-pole breaker for clarity. The popped dryer breaker is clearly a single pole, as you can see the one entry slot. It's apparently connected to the left bus and only that bus.

If I understand your description correctly, the only flaw here is the handles of each leg need to be tied together so that if one leg trips, the other side is shut off as well. I would like to see a picture of the installation before making a final determination.
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bcworkzMay 26 '14 at 2:21

Added photo of the breakers. The top tandem breaker has its neutral attached to the neutral bar, but I can't show that since that space is an unholy mess.
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Kurt GranrothMay 26 '14 at 5:00

5 Answers
5

It sounds like you may be mistaken as to how this is wired, or that perhaps I'm just not understanding your explanation. As others have mentioned, it's not possible to get 240 volts from a single pole in a 120/240V split phase system. Each tandem breaker provides 2 120 V circuits, this is true. However, if you measure between the terminals on a single tandem breaker, you'll get 0 volts. This is because the terminals are both powered from the same leg, and so are at the same voltage potential. If you measure from a terminal on the top tandem breaker to a terminal on the bottom one, then you'll measure 240 volts. This is because each breaker is connected to a different leg, which are each one half of a 240 volt circuit.

With all that said. For this setup to work, one appliance would have to be connected to both breaker. Something like this...

Notice that each appliance circuit has one wire connected to each of the tandem breakers. In this situation, you'd need a device like Speedy Petey shows.

Which ties the breaker handles together, to provide common trip characteristics.

Notice how the inner handles are tied together, and that the outer handles are also tied to each other. This way if either trip (or are turned off by the user), the entire circuit is shut off.

If this is wired the way you've explained, where the dryer is connected to the top tandem and the heater is connected to the bottom. Then there's some magic going on in those breakers.

You are absolutely right! The dryer takes the top-most and bottom-most switches with the water heater taking the middle two. That's absolutely good to know because I'm going to be replacing those with two full double-pole breakers and it looks like I would have wired them incorrectly (at least at first)! It finally makes sense why this works in the first place. Thanks!
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Kurt GranrothMay 28 '14 at 1:52

I wonder if anyone makes a product that would take two breaker slots and provide two side-by-side pairs of outputs, using an internal cross-over? Having the inner wires feed one device and the outer feed another seems really hokey.
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supercatJan 23 at 17:26

I have read that tandem breakers can be mounted to either feed power from one leg of the bus bar or from two different legs. If it's the latter in your case, then you'd be getting 240V after all. Check the terminals of the breakers in question (or check at the outlets where your appliances are plugged in) with a voltmeter, if you haven't already, and confirm what is actually being fed to your appliances.

I'll pop one of those tomorrow to see if they are on two legs or not, but I'm fairly certain that they aren't. As far as I know, only some GE panels have tandem slots that have two buses in one slot. I'll verify, though. I am definitely getting 240V.
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Kurt GranrothMay 26 '14 at 5:09

In US household electrical service, you CAN'T get 240V from a single pole. If you have 240V across them, the breaker is straddling both busses. You just can't get 240v from the one bus.

The only issue is connecting the two switch handles. Obviously a nail or piece of wire will work but it looks shoddy. There actually is a real product for this, called a circuit breaker handle tie. I just bought some myself at the local big orange home improvement store to replace a piece of wire stuck through one of mine.

On his particular breaker, it's a non-separatable tandem so this pin won't work. I've dealt with these for paired slim breakers on other brands, insert the barrel and install both breakers at once.
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Fiasco LabsMay 26 '14 at 17:39

Modified to match to new provided information. Speedy Petey's answer contains the circuit breaker you need to install to correct the situation. As you can see from the breaker inter-ties, they basically take two of these breakers and manually finish off what the installer was attempting to do in a safe manner.

Murray MH-T MP3030 (this unit gets its power from one leg to produce two 120V circuits)

The setup is working because each breaker pair feeds off the proper panel bus leg (One attaches to L1, the other to L2). From your added information, each 240V circuit is split across the two breakers to do this.

What's Wrong -- The major safety concern here is that with a circuit fault, only one of the breakers in the set may trip, leaving the 240V circuit partially energized. For instance if the dryer heating coil burns through and one end touches ground, that side will trip, leaving the other leg hot. You definitely want a circuit fault to totally power down both legs.

Also related is the fact that anyone not paying attention to the panel setup may power off the circuit to work on it and not realize they switched off adjacent pairs instead of alternate pairs of each 240V circuit, leaving a hot leg in the circuit they think is powered down. You want things to be fail-safe. This isn't fail-safe, but is a booby trap waiting for the inattentive or hurried. NEC frowns heavily on things like this.

Murray bus backplane with interleaved fingers so adjacent full size breakers are on opposite legs. Not true for the MH-T MP3030 breaker pair, each breaker in the set is half-width and the combination is fed off only one leg blade.

The independent switches is definitely a problem and yes, that's how I discovered this was a problem. I flipped one of the switches and was very shocked (pun sadly appropriate) that the power was still on. Combining the two switches (maybe with a dowel or nail) should fix that. Is that the only problem, though? I'm pretty sure that this is still a single-pole breaker and that both of them are on the same bus. A double-pole definitely balances the load better than something like this, but if that's the only reason, then why isn't this a common use case for space starved panels?
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Kurt GranrothMay 26 '14 at 5:07

1

You wouldn't be getting 240V if they were both on the same bus Leg, 240V is created by the additive combination of +120V from L1 and -120V from L2. If you were to pull the breakers out, you probably find this is one of those designs where the power bus tabs alternate so adjacent breakers are on separate legs. In a dryer, the motor and controls are run from one leg at 120V between the leg and Neutral and the heating element takes the full 240V between L1 and L2. And, yes, I've seen an appropriate sized nail run through the holes in the handle to pair the breakers for 240V use.
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Fiasco LabsMay 26 '14 at 17:31

I added a photo which shows that the breaker is, indeed, only on one bus leg. This breaker only has one slot. It's also definitely producing 240v, since my dryer works like a charm. Tandems are made to produce two 120v circuits from one bus. Perhaps the real problem here is that the TWO circuits are producing the 240v, but since they are split, the neutral only applies to one of them?
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Kurt GranrothMay 27 '14 at 2:22

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Neutral is neutral, even if the 240 legs are taken off alternate breakers. Wow, I see what you mean on that setup now, I couldn't see from the picture that the breaker pair is only fed by one leg. I didn't quite get the description and it sounded like adjacent breakers were being used to produce 240V. Using alternate breakers to produce 240V is damn dangerous if you need to be sure the power is cut and make the wrong assumption.
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Fiasco LabsMay 27 '14 at 3:10